The long-term goal of our laboratory is to understand the structure and function of a variety of redundant genes in the fruit fly, Drosophila melanogaster, by both genetic and physical means. By means of restriction enzymes, heteroduplex mapping with the electronmicroscope and Tm measurements, we have demonstrated that a very high degree of homology exists between the non-transcribed spacers of the ribosomal RNA genes in a number of species closely related to Drosophila melanogaster. This is in marked contrast to the situation that exists in Xenopus. Experiments are also being conducted in order to obtain the complete tandem array of genes coding for 5S RNA, including those sequences at the ends of this tandem array which connect to the flanking genes. In this case, we are interested in determining if there are specific sequences that lie at the boundaries (interbands) of chromomeres. Finally, we are attempting to understand the molecular basis controlling position effect variegation. Experiments are outlined to determine if there is a causal relationship between the ability to generate a variegating phenotype and the rearrangement of compensatory response (cr ion) loci.